If you just want to quote the entire post, click the Quote button and add your reply at the end. (After the closing quote tag).

If you just want to quote part of it, again click Quote, but you now have some work to do. Using the mouse in the reply, highlight the parts you don't want to quote, and delete them. Before publishing it, make sure that you end up with just one opening and one closing tag round what you want to quote, and make sure your reply is after the closing tag. I'd also advise previewing it to make sure it looks as you intended. Two examples follow. To see what the original text looks like I've followed them with the same examples with the extra spaces between the [ and ] removed.

[ quote ]The bit you want to quote anonymously.[ /quote ]

[ quote="JoeBloggs" ]If you want it to say "JoeBloggs wrote" rather than just have a single " at the start.[ /quote ]

The bit you want to quote anonymously.

JoeBloggs wrote:If you want it to say "JoeBloggs wrote" rather than just have a single " at the start.

NOTE: If you attempt to edit the tags and get it wrong, it will not look correct and I will - yet again - have to edit it to correct the errors.